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United States Patent |
5,783,885
|
Post
|
July 21, 1998
|
Self-adjusting magnetic bearing systems
Abstract
A self-adjusting magnetic bearing automatically adjusts the parameters of
an axially unstable magnetic bearing such that its force balance is
maintained near the point of metastable equilibrium. Complete
stabilization can be obtained with the application of weak restoring
forces either from a mechanical bearing (running at near-zero load, thus
with reduced wear) or from the action of residual eddy currents in a
snubber bearing. In one embodiment, a torque is generated by the approach
of a slotted pole to a conducting plate. The torque actuates an assembly
which varies the position of a magnetic shunt to change the force exerted
by the bearing. Another embodiment achieves axial stabilization by sensing
vertical displacements in a suspended bearing element, and using this
information in an electrical servo system. In a third embodiment, as a
rotating eddy current exciter approaches a stationary bearing, it heats a
thermostat which actuates an assembly to weaken the attractive force
between the two bearing elements. An improved version of an
electromechanical battery utilizing the designs of the various embodiments
is described.
Inventors:
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Post; Richard F. (Walnut Creek, CA)
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Assignee:
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The Regents of the University of California (Oakland, CA)
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Appl. No.:
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511981 |
Filed:
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August 7, 1995 |
Current U.S. Class: |
310/90.5; 310/80; 310/103; 310/105 |
Intern'l Class: |
H02K 007/09; H02K 007/06; H02K 049/00 |
Field of Search: |
310/90.5,103,105,80,104
|
References Cited
U.S. Patent Documents
3611815 | Oct., 1971 | Fischell | 74/5.
|
3779618 | Dec., 1973 | Soglia et al. | 310/90.
|
3845997 | Nov., 1974 | Boden et al. | 308/10.
|
3877761 | Apr., 1975 | Boden et al. | 308/10.
|
3929390 | Dec., 1975 | Simpson | 310/90.
|
3976339 | Aug., 1976 | Sabnis | 310/90.
|
4043614 | Aug., 1977 | Lyman | 308/10.
|
4090745 | May., 1978 | Dohogne et al. | 310/90.
|
4398773 | Aug., 1983 | Boden et al. | 308/10.
|
4405286 | Sep., 1983 | Studer | 417/1.
|
4585282 | Apr., 1986 | Bosley | 310/90.
|
4620752 | Nov., 1986 | Fremerey et al. | 310/90.
|
5331819 | Jul., 1994 | Matsuda et al. | 62/51.
|
5386166 | Jan., 1995 | Reimer et al. | 310/90.
|
5495221 | Feb., 1996 | Post | 335/299.
|
5563565 | Oct., 1996 | Hull | 335/216.
|
Primary Examiner: Stephan; Steve L.
Assistant Examiner: Wallace, Jr.; Michael J.
Attorney, Agent or Firm: Wooldridge; John P.
Goverment Interests
The United States Government has rights in this invention pursuant to
Contract No. W-7405-ENG-48 between the United States Department of Energy
and the University of California for the operation of Lawrence Livermore
National Laboratory.
Claims
I claim:
1. A self-adjusting magnetic bearing, comprising:
a stationary bearing element;
a rotatable bearing element magnetically levitated by said stationary
bearing element;
wherein said self-adjusting magnetic bearing is intrinsically radially
stable; and
active means for automatically adjusting the magnetic parameters of said
stationary bearing element with respect to said rotatable bearing element,
resulting in axial stability, such that the force balance of said
self-adjusting magnetic bearing is maintained near the point of metastable
equilibrium,
wherein said stationary bearing element comprises at least one first
annular iron pole piece
wherein said rotatable bearing element comprises at least one second
annular iron pole piece fixedly attached to an annular conducting plate
comprising non-magnetic material,
wherein said at least one first annular pole piece and said at least one
second annular pole piece are configured to exert magnetically attractive
forces upon each other such that said rotatable bearing element tends to
be pulled toward said stationary bearing element, and
wherein said means for automatically adjusting the magnetic parameters
comprises a stationary drive screw supporting an assembly comprising a
magnetic shunt pair with an internal thread, each shunt of said shunt pair
supporting a slotted pole.
2. The self-adjusting magnetic bearing of claim 1, wherein said annular
conducting plate comprises copper.
3. The self-adjusting magnetic bearing of claim 1, further comprising:
an eddy-current damper plate fixedly attached to said rotatable bearing
element; and
an annular permanent magnet element fixedly attached to said stationary
bearing element and having field lines that intersect said eddy current
damper plate.
4. The self-adjusting magnetic bearing of claim 3, wherein said eddy
current damper plate comprises a plurality of concentric conducting rings
separated by material having a conductivity that is less than that of said
conducting rings.
5. The self-adjusting magnetic bearing of claim 3, wherein said eddy
current damper plate comprises a non-magnetic metal plate having deep
circular grooves, wherein only a thin web of supporting material remains
on the surface of said eddy current damper plate that is farthest from
said annular permanent magnet element.
6. The self-adjusting magnetic bearing of claim 1, further comprising:
an eddy-current damper plate fixedly attached to said stationary bearing
element; and
an annular permanent magnet element fixedly attached to said rotatable
bearing element and having field lines that intersect said eddy-current
damper plate.
7. The self-adjusting magnetic bearing of claim 1, further comprising a
vacuum enclosure, wherein said stationary bearing element is located
outside said vacuum enclosure and wherein said rotatable bearing element
is located inside said vacuum enclosure.
8. The self-adjusting magnetic bearing of claim 1, further comprising:
an eddy-current damper plate fixedly attached to said rotatable bearing
element; and
an annular permanent magnet element fixedly attached to said stationary
bearing element and having field lines that intersect said eddy-current
damper plate.
9. The self-adjusting magnetic bearing of claim 8, wherein said
eddy-current damper plate comprises a plurality of concentric conducting
rings separated by material comprising with a conductivity that is less
than that of said conducting rings.
10. The self-adjusting magnetic bearing of claim 8, wherein said
eddy-current damper plate comprises a non-magnetic metal plate having deep
circular grooves wherein only a thin web of supporting material remains on
the surface farthest from said annular permanent magnet element.
11. The self-adjusting magnetic bearing of claim 1, further comprising a
vacuum enclosure, wherein said stationary bearing element is located
outside said vacuum enclosure and wherein said rotatable bearing element
is located inside said vacuum enclosure.
12. A self-adjusting magnetic bearing, comprising:
a stationary bearing element;
a rotatable bearing element magnetically levitated by said stationary
bearing element;
wherein said self-adjusting magnetic bearing is intrinsically radially
stable; and
means for automatically adjusting the magnetic parameters of said
stationary bearing element with respect to said rotatable bearing element,
resulting in axial stability, such that the force balance of said
self-adjusting magnetic bearing is maintained near the point of metastable
equilibrium,
wherein said stationary bearing element comprises:
a cup-shaped element having a central axis; and
an interior magnetic bearing element comprising a pole having permanent
magnet material, wherein said interior magnetic bearing element is fixedly
attached to said central axis of said cup shaped element,
wherein said rotatable bearing element comprises an iron disc, and
wherein said means for automatically adjusting the magnetic parameters
comprise:
a pole assembly detached from and located between said stationary bearing
element and said rotatable bearing element, said pole assembly comprising
a disc having a center iron piece and at least one alternating concentric
ring of non-magnetic metal and iron, said pole assembly attached to a
support structure with a spring;
means for sensing vertical displacements away from equilibrium of said
rotatable bearing element; and
a control coil surrounding said interior magnetic bearing element, wherein
a feed-back circuit is electrically connected between said means for
sensing vertical displacements and said control coil.
13. The self-adjusting magnetic bearing of claim 12, wherein said means for
sensing vertical displacements comprises a strain gauge operatively
connected to said spring.
14. The self-adjusting magnetic bearing of claim 13, further comprising:
an eddy-current damper plate fixedly attached to said rotatable bearing
element; and
an annular permanent magnet element fixedly attached to said stationary
bearing element and having field lines that intersect said eddy-current
damper plate.
15. The self-adjusting magnetic bearing of claim 14, wherein said
eddy-current damper plate comprises a plurality of concentric conducting
rings separated by material comprising with a conductivity that is less
than that of said conducting rings.
16. The self-adjusting magnetic bearing of claim 14, wherein said
eddy-current damper plate comprises a non-magnetic metal plate having deep
circular grooves wherein only a thin web of supporting material remains on
the surface farthest from said annular permanent magnet element.
17. The self-adjusting magnetic bearing of claim 14, wherein said eddy
current damper plate comprises a plurality of concentric conducting rings
separated by material having a conductivity that is less than that of said
conducting rings.
18. The self-adjusting magnetic bearing of claim 14, wherein said eddy
current damper plate comprises a non-magnetic metal plate having deep
circular grooves, wherein only a thin web of supporting material remains
on the surface of said eddy current damper plate that is farthest from
said annular permanent magnet element.
19. The self-adjusting magnetic bearing of claim 13, further comprising a
vacuum enclosure, wherein said stationary bearing element is located
outside said vacuum enclosure and wherein said rotatable bearing element
is located inside said vacuum enclosure.
20. The self-adjusting magnetic bearing of claim 13, further comprising:
an eddy-current damper plate fixedly attached to said stationary bearing
element; and
an annular permanent magnet element fixedly attached to said rotatable
bearing element and having field lines that intersect said eddy-current
damper plate.
21. A self-adjusting magnetic bearing, comprising:
a stationary bearing element;
a rotatable bearing element magnetically levitated by said stationary
bearing element;
wherein said self-adjusting magnetic bearing is intrinsically radially
stable; and
means for automatically adjusting the magnetic parameters of said
stationary bearing element with respect to said rotatable bearing element,
resulting in axial stability, such that the force balance of said
self-adjusting magnetic bearing is maintained near the point of metastable
equilibrium,
wherein said stationary element comprises:
a first cup-shaped element comprising magnetically conducting material and
having a first central axis;
a second cup-shaped element comprising magnetically conducting material and
having a second central axis;
a permanent magnet piece fixedly connected to said first central axis and
said second central axis to fixedly connect said first cup to said second
cup;
a third cup-shaped element comprising a third central axis, said third cup
shaped element comprising magnetically conducting material and
non-magnetic metallic material with at least two through holes; and
a copper eddy current plate supported by two support rods that extend from
said copper eddy current plate through said at least two through holes and
are fixedly attached to said second cup shaped element;
wherein said rotatable bearing element comprises:
an eddy-current exciter; and
a pole assembly fixedly attached to said eddy current exciter;
wherein said means for automatically adjusting the magnetic parameters
comprise a thermostat element fixedly supported between said copper eddy
current plate and said non-magnetic metallic material of said third cup
shaped element.
22. The self-adjusting magnetic bearing of claim 21, wherein said
eddy-current exciter comprises a slotted pole.
23. The self-adjusting magnetic bearing of claim 22, further comprising at
least one tension spring fixedly attached between said third cup shaped
element and said eddy current plate.
24. The self-adjusting magnetic bearing of claim 21, wherein said
eddy-current exciter comprises a planar Halbach array.
25. The self-adjusting magnetic bearing of claim 21, wherein said
thermostat element is selected from a group consisting of a sealed sylphon
bellows and an assembly comprising a bimetallic element, wherein said
sealed sylphon bellows comprises a vaporizable liquid.
26. The self-adjusting magnetic bearing of claim 21, further comprising:
an eddy-current damper plate fixedly attached to said rotatable bearing
element; and
an annular permanent magnet element fixedly attached to said stationary
bearing element and having field lines that intersect said eddy-current
damper plate.
27. The self-adjusting magnetic bearing of claim 26, wherein said
eddy-current damper plate comprises a plurality of concentric conducting
rings separated by material comprising with a conductivity that is less
than that of said conducting rings.
28. The self-adjusting magnetic bearing of claim 26, wherein said eddy
current damper plate comprises a non-magnetic metal plate having deep
circular grooves wherein only a thin web of supporting material remains on
the surface farthest from said annular permanent magnet element.
29. The self-adjusting magnetic bearing of claim 21, further comprising a
vacuum enclosure, wherein said stationary bearing element is located
outside said vacuum enclosure and wherein said rotatable bearing element
is located inside said vacuum enclosure.
30. The self-adjusting magnetic bearing of claim 21, further comprising:
an eddy-current damper plate fixedly attached to said stationary bearing
element; and
an annular permanent magnet element fixedly attached to said rotatable
bearing element and having field lines that intersect said eddy-current
damper plate.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to magnetic bearings, and more specifically,
it relates to magnetic bearings that are intrinsically radially stable and
are automatically stabilized axially.
2. Description of Related Art
U.S. Pat. No. 4,043,614 discloses a magnetic suspension system for
efficiently supporting a rotor of relatively large radius and high
available moment of inertia. It includes concentric stator and rotor
elements spaced apart by narrow gaps of relatively large diameter and very
small axial extent. A pair of spaced discs in the stator are oppositely
polarized by a permanent magnet, their peripheral magnetic field strength
being selectively augmented or diminished in predetermined sectors by
electromagnet windings thereon. The rotor includes narrow ring faces
juxtaposed to said discs and a permanent magnetic polarization being
provided between the narrow ring faces. Means are provided for varying the
relative electromagnetic contributions in the different sectors to
maintain stability of rotor positioning with minimized electric power.
U.S. Pat. No. 3,877,761 provides a contact-free bearing system for radially
supporting a rotor, rotatable at high speed, with respect to a stationary
member. It comprises a magnetic system including electromagnetic bearings.
The electromagnetic bearings includes an endless ferromagnetic core
carrying a spiral or toroidal winding which includes or acts as at least
three separate coils. Electrical signals, dependent upon the radial
deviation in the position of the rotor from a predetermined position are
applied to said winding such that the coils produce magnetic fields which
are applied to the rotor and are of different magnitude or direction.
U.S. Pat. No. 3,845,997 provides a magnetic bearing assembly for
journalling a rotor which is at least partly ferromagnetic in a stator,
the bearing assembly being able to absorb transverse forces acting on the
rotor. The assembly includes radial bearing means holding the rotor on a
desired axis of rotation and at least one magnet. An air gap is defined
between the rotor and the magnet in which a magnetic field is set up. The
magnetic field has a first magnetic field component which is constant
around the periphery of the air gap and on which is superimposed a second
magnetic field component which varies around the periphery of the air gap
to absorb the transverse forces.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide self adjusting magnetic
bearings.
It is also an object of the present invention to minimize the losses in a
self-adjusting magnetic bearing such that the force equilibrium position
of a suspended element is at the point where the repulsive force is
minimal, so that eddy-current induced forces need only preserve stability
in the close vicinity of the position of vertical force equilibrium.
An automatic means for adjusting the parameters of an axially unstable
magnetic bearing is provided. The force balance of the magnetic bearing is
maintained near the point of metastable equilibrium where complete
stabilization can be obtained with the application of weak restoring
forces, either from a mechanical bearing or from the action of residual
eddy currents in a snubber bearing.
In a first embodiment, a repulsive force is generated by the approach of a
slotted pole surface to a conducting plate (or a conducting cylinder).
Eddy currents thus generated will also generate a torque in the conducting
plate, with a magnitude that is proportional to the force of repulsion.
The torque is used to actuate an upper assembly to rotate about a drive
screw to vary the position of a magnetic shunt pair within the magnetic
bearing. This shunt then changes the force exerted by the bearing until it
approaches a position of force equilibrium. The eddy current-driven
actuator thus performs a similar function to the electronic feedback
circuits used in "active" magnetic bearings to maintain the bearing at the
point of metastable equilibrium.
A second embodiment achieves axial stabilization through a simplified
electrical servo system. A detached pole assembly facilitates the
stabilization of a rotor against transverse "whirl" instabilities through
the use of damping and/or spring-like elements that support that assembly.
That same element is used as a sensor for vertical displacements away from
equilibrium of the lower bearing element that supports the rotor. A
vertical displacement, up or down, of the lower pole element will be
reflected in a change in the force balance on the intermediate pole
element. This change may result in a vertical displacement of that
element, acting against supporting spring-like elements. This displacement
can then be sensed, and used in an electronic feedback loop to maintain a
stable equilibrium.
In a third embodiment, a rotating eddy current exciter induces heat in a
stationary eddy current plate. The heat causes a thermostat element to
expand which weakens the levitational force between a stationary and a
rotatable bearing element. The design is made such that the net resultant
of the attraction and of the repulsion, taken together with the weight
that is being supported (e.g., the rotor of an electromechanical battery)
is a state of vertical force equilibrium. It is further arranged, by
geometry, and, if indicated, by the use of the "reduced derivative"
concept that the position of vertical force equilibrium is a stable one.
A vibration damper is disclosed for use in high-speed rotating systems,
particularly ones that use magnetic bearings such as the ones described
herein. In such systems it is common to use what are called "eddy-current
dampers" in which a stationary conducting plate (copper, for example) is
located close to the surface of a rotating annular permanent magnet. This
magnet is often an integral part of the magnetic bearing system itself.
Incorporating the designs of the disclosed embodiments, an improved version
of a type of modular electromechanical (EM) battery is described. This
much-simplified version is particularly well suited for stationary
applications, where low cost, high reliability, and long service life are
the critical parameters. This version involves a major change in the
magnetic bearing design, particularly its elements within the vacuum
chamber. This change in bearing design results in an overall design that
permits the simplification of all the elements of the EM battery that
operate within the vacuum housing.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an embodiment using eddy currents to drive a shunt assembly.
FIG. 2 shows an embodiment using the position or strain on an intermediate
pole assembly to drive a control coil.
FIG. 3A shows a thermally controlled self-adjusting magnetic bearing.
FIG. 3B shows the relative rotation of magnetization of an eddy current
exciter in a Halbach array configuration for use in an embodiment of the
thermally controlled self-adjusting magnetic bearing of FIG. 3A.
FIG. 3C shows a top view of a Halbach eddy current exciter with a
representative pie shaped magnet piece for use in an embodiment of the
thermally controlled self-adjusting magnetic bearing of FIG. 3A.
FIG. 4 shows an improved eddy-current damper for use in the embodiments of
FIGS. 1-3 and 5.
FIG. 5 shows the use of the embodiments of FIGS. 1-4 in an improved modular
electromechanical battery.
DETAILED DESCRIPTION OF THE INVENTION
The invention is an automatic means for adjusting the parameters of an
axially unstable magnetic bearing such that its force balance is
maintained near the point of metastable equilibrium, at which point
complete stabilization can be obtained with the application of only weak
restoring forces, either from a mechanical bearing (running at near-zero
load, thus with reduced wear) or from the action of residual eddy currents
in a snubber bearing. Generally, this self-adjusting magnetic bearing is
intrinsically radially stable and has a stationary bearing element and a
rotatable bearing element that is magnetically levitated by the stationary
bearing element. The bearing includes a means for automatically adjusting
the magnetic parameters of the stationary bearing element with respect to
the rotatable bearing element to provide axial stability, such that the
force balance of the self-adjusting magnetic bearing is maintained near
the point of metastable equilibrium.
Referring to the cross sectional view shown in FIG. 1, an annular
stationary bearing element 10 comprising annular iron pole pieces 12 is
attached to a support structure (not shown). Annular pole pieces 12 are
separated with permanent magnet material 13. A generally disc shaped
rotatable bearing element 14 is magnetically levitated by the stationary
bearing element 10. Rotatable element 14 comprises annular iron pole
pieces 16 which are attached to a rotatable disc 18. Pole pieces 16 are
located directly across from the iron pole pieces 12 of the stationary
element 10. Pole pieces 12 and pole pieces 16 are selected to exert
magnetically attractive forces upon each other so that rotatable bearing
element 14 tends to be pulled toward stationary upper bearing element 10.
An annular conducting plate 20 is attached to the outer annular pole piece
16 of the rotatable bearing element 14. In this embodiment, the conducting
plate 20 is made of copper, but may be made of any non-magnetic
electrically conductive material. A stationary drive screw 22 is attached
to a supporting structure (not shown), and supports a rotatable assembly
24, which has an internal thread which matches the thread of stationary
drive screw 22. Rotatable assembly 24 comprises a shunt pair 26 and a
slotted pole pair 28 or a planar Halbach array 28. Rotatable bearing
element 14 typically supports a rotor and is rotated by a drive mechanism
(not shown).
In operation, a repulsive force is generated by the approach of slotted
pole surface 28 to conducting plate (or a conducting cylinder) 20. Eddy
currents thus generated will also generate a torque in the conducting
plate 20, with a magnitude that is proportional to the force of repulsion.
The torque is used to actuate rotatable assembly 24 to rotate about drive
screw 22 to vary the position of the magnetic shunt pair 26 within the
permanent-magnet-excited magnetic bearing. This shunt then weakens (or
strengthens) the force exerted by the bearing until it approaches a
position of force equilibrium, i.e. a position of metastable equilibrium.
In this action the eddy current-driven actuator performs a similar
function to the electronic feedback circuits used in "active" magnetic
bearings to maintain the bearing at the point of metastable equilibrium.
It should be recognized that the residual repulsive force from eddy
currents exerted by a snubber bearing itself can be used to insure a
stable equilibrium. The self-adjusting action can be used to insure that
the energy losses and heating effects associated with those eddy currents
are automatically reduced to a minimal value, since with proper design the
system will always drive itself to a position of vanishing torque on the
actuator, and thus to vanishingly small eddy-current losses.
Referring again to FIG. 1, the shunt pair 26 is only actuated in a
direction to weaken the field, that is, it would drive the lifting force
down only. This would be acceptable if the magnetic bearing had excess
lifting capacity that would only need a one-time adjustment to bring the
force down to near-equilibrium (as in stationary applications, for
example). For systems where one would need both positive and negative
excursions about equilibrium, one would employ a more complicated drive
system, i.e. one with eddy current discs on both upper and lower sides of
the azimuthally segmented poles (or vice-versa). In such an embodiment,
drive screw 22 is hollow. A second eddy current disc is then located above
the shunt pair. This second eddy current disc is supported by a drive
shaft that is fixedly connected to the rotating disc 18. The drive shaft
passes through the hollow drive screw 22 to connect the rotating disc to
the second eddy current disc. A second pair of slotted pole pieces or a
planar Halbach array pair are attached with a reversing gear to the shunt
pair 26. In operation, as eddy current plate 20 approaches slotted pole
pair 28, a torque is induced which drives the shunt pair 26 down to a
position closer to annular pole pieces 12, thus reducing the attractive
force exerted by annular pole pieces 12 on annular pole pieces 16,
allowing the rotatable bearing element 14 to move away from the stationary
bearing element 10. As the rotatable bearing element 14 moves away from
the stationary bearing element 10, a point is reached where the force
exerted by the second eddy current plate on the second slotted pole pair
exceeds the force exerted by the eddy current plate 20 on the slotted pole
pair 28, thus driving the shunt pair 26 away from annular pole pieces 12.
This causes the force exerted by annular pole piece 12 on annular pole
piece 16 to increase, thus driving the system to a new position of force
equilibrium.
A second embodiment, shown in the cross-sectional view of FIG. 2, presents
another means for achieving axial stabilization, accomplished through a
much-simplified electrical servo system. A detached pole assembly 30
facilitates the stabilization of the rotor 32 against transverse "whirl"
instabilities through the use of damping and/or spring-like elements 39
that support that assembly. Detached pole assembly 30 is used as a sensor
for vertical displacements away from equilibrium of the bearing element 34
that supports the rotor. A vertical displacement, up or down, of the lower
pole element 34 will be reflected in a change in the force balance on the
detached pole assembly 30. This change may result in a vertical
displacement of the detached pole assembly 30, acting against supporting
spring-like elements. This displacement can then be sensed, and used in an
electronic feedback loop to maintain a stable equilibrium. If the springs
are reasonably compliant, this sensing might be accomplished simply
through the change in electrical capacity between the detached pole
assembly 30 and its surroundings (the stationary bearing element 36,
and/or a chamber wall). Alternatively, if the supporting spring elements
are very stiff in the axial direction then the change in force on the
detached pole assembly 30 associated with a vertical displacement of the
lower bearing element 34 could be sensed by resistive-type strain gauge
elements.
The self-adjusting magnetic bearing shown in FIG. 2 has a stationary cup
shaped element 36 having a central axis and an interior magnetic bearing
element 37 with a pole having permanent magnet material. Magnetic bearing
element 37 is fixedly attached to the central axis of the stationary cup
shaped element 36. The rotatable bearing element 34 may be an of iron
disc. In this embodiment, the means for automatically adjusting the
magnetic parameters comprise the detached pole assembly 30 located between
the stationary bearing element 36 and the rotatable bearing element 34.
The detached pole assembly 30 is a disc with a center iron piece 31 and at
least one alternating concentric ring of non-magnetic metal 33 and iron
35. Detached pole assembly 30 is attached to a support structure with a
spring 39 (shown in box form) with means for sensing vertical
displacements away from equilibrium of the rotatable bearing element 34.
Control coil 38 surrounds the interior magnetic bearing element 37 and a
feed-back circuit (not shown) is electrically connected between the means
for sensing vertical displacements 39 and the control coil 38.
The use of a reduced-derivative design in the stationary bearing element
36, together with the slow response of a rotor 32 to the acceleration of
gravity (in the vicinity of force equilibrium), has a favorable impact on
the design of the electronic servo circuitry. Because of this slow
response, the servo system need have only a very limited frequency
response (bandwidths of a few tens of Hertz). Thus, very simple and
inexpensive components should be adequate, and in most cases it should not
be necessary to laminate the iron in the magnet poles, thus reducing their
cost. The upper bearing element 36 may be located above and outside of a
vacuum chamber wall 40, so that the magnetic attractive forces are exerted
through the chamber wall itself.
FIG. 3A shows a sectional view of a thermally adjusting magnetic bearing.
Stationary bearing element 50 comprises a cup shaped element 52 made of
magnetically conducting material such as iron. Another cup shaped element
54 of stationary element 50 also comprises magnetically conducting
material. A permanent magnet piece 58 is connected between the first cup
52 the second cup 54. A non-magnetic metallic piece 56 such as aluminum
may be included between the two cup shaped elements 52 and 54. Another cup
shaped element 60 made of magnetically conducting material 61 and
non-magnetic metallic material 66 with two through holes is nested within
cup shaped element 54. An eddy current plate 68 is supported by two
support rods 62, 64 that extend from the eddy current plate 68 through the
two through holes and are fixedly attached to the cup shaped element 54.
Rotatable bearing element 70 is has an eddy current exciter 72 an a pole
assembly 76 fixedly attached thereto. The means for automatically
adjusting the magnetic parameters comprise a thermostat element 65
supported between the eddy current plate 68 and the non-magnetic metallic
material 66 of cup shaped element 54. The eddy current exciter 72 may be a
slotted pole or a Halbach array, as shown in FIGS. 3B and 3C. Thermostat
element 65 may be a sealed sylphon bellows with a vaporizable liquid or
element 65 may be an assembly having a bimetallic element. At least one
tension spring 78 may be fixedly attached between the cup shaped element
60 and the eddy current plate 68. Rotatable bearing element 70 is
generally disc shaped and may have an annular aluminum disc 74 attached
thereto. Annular poles 76 may be attached to disc 74. Rotatable bearing
element 70 is magnetically levitated by stationary bearing element 50, due
to a magnetically attractive relationship therebetween.
In FIG. 3, the rotating eddy current exciter 72 exerts a repelling force
(in opposition to the attractive force of the upper poles) on the
stationary eddy current plate 68. The design is made such that the net
resultant of the attraction and of the repulsion, taken together with the
weight that is being supported (e.g., the rotor of an electromechanical
battery) is a state of vertical force equilibrium. It is further arranged,
by geometry, and, if indicated, by the use of the "reduced derivative"
concept that the position of vertical force equilibrium is a stable one.
FIG. 3B shows the relative rotation of magnetization of an eddy current
exciter 72 in a Halbach array configuration. FIG. 3C shows a top view of a
Halbach eddy current exciter with a representative pie shaped magnet piece
73.
Because the force derivative in the transverse direction of the eddy
current plate/eddy current exciter system can be made to be small by
design, whereas the force derivative in the transverse direction of the
attractive bearing is strong and in the stable (centering) direction, the
combination of the two represents a passively stable system. This
stability will be maintained over a finite range of vertical displacements
(as dictated by the particular design) as long as the (stabilizing) force
derivative of the eddy current elements is larger than the destabilizing
force derivative of the magnetic bearing, and the resultant vertical force
equals the downward gravitational force on the suspended mass.
By weakening the attractive force of the stationary bearing element 50, for
example by an adjustment to the strength of the permanent magnet 54, a
widening of the gap between the stationary element 50 and the rotatable
element 70 will result, which in turn will result in a decrease in the
repulsive force on the eddy current exciter 72, leading to a new and
stable position of force equilibrium. But it can also be readily seen that
the amount of power dissipated in the eddy current plate 68 is thus
reduced, so that the power losses of eddy current origin in the bearing
system are reduced.
Note now that when the rotating eddy current exciter 72 is positioned close
to the surface of the eddy current plate 68, heat will be generated in the
eddy current plate 68, causing the thermostat element 65 to expand
vertically, stretching springs 78. Since the eddy current plate 68 is
restrained in position by the support rods 62 and 64, the expansion of the
thermostat element 65 necessarily results in an upward motion of the
movable pole of cup shaped element 60. This motion in turn weakens the
attractive force of the upper stationary poles of stationary element 50 on
the lower rotating poles of rotatable element 70, resulting in a lowering
of the rotating parts to a new equilibrium position, one in which the
heating is less. With care in the design the end result is that the eddy
current losses are minimized, while still maintaining a stable
equilibrium, both axially and radially.
It may turn out to be advantageous to have a similar (but not necessarily
identical) bearing assembly located lower down on the rotor, so that the
position of force equilibrium is one that is determined by the vector sum
of both bearing forces, taken together with the supported mass. In this
case the design should be such as to locate the force equilibrium position
of the two magnetic pole systems at a point where the amount of oppositely
acting eddy current (repulsive) force is minimal, so that the
eddy-current-induced forces need only to preserve stability in the close
vicinity of the position of vertical force equilibrium.
It is not necessary that the system described be absolutely stable in the
steady-state sense. If the condition on the vertical force derivative is
satisfied over a reasonable range, then the system might slowly oscillate
vertically with small amplitude, on a thermal time scale (i.e. minutes)
without seriously affecting the operation of the system. The system
described herein is much simpler to implement, and reliable compared to
most "active" magnetic bearings (using sensors and electronic servo
circuits) that this level of instability should be ignorable as compared
to the economic advantage that is gained.
When the bearings are used in an evacuated region, as in most
electromechanical battery (EMB) designs, the heat balance that determines
the temperature of the eddy current plate 68 is primarily a balance
between eddy current heating and radiation losses (assuming that the
support rods 62 and 64 are made of a low conductivity material such as
stainless steel). This being the case it only will take a watt or so in a
typical-sized system to raise the temperature of the eddy current plate 68
by many tens of degrees Celsius. It should thus be readily possible to
design the system so that in steady state the losses in eddy current
plates are less than a watt. For an electromechanical battery (EMB)
storing about 5 kilowatt-hours, a watt of bearing losses, taken alone,
would lead to a rundown time constant on the order of 5000 hours, which
would be of essentially no consequence.
FIG. 4 shows an improved version of a vibration damper for use in
high-speed rotating systems, particularly ones that use magnetic bearings.
In such systems it is common to use what are called "eddy-current dampers"
in which a stationary conducting plate (copper, for example) 80 is located
close to the surface of a rotating annular permanent magnet 82. This
magnet is often an integral part of the magnetic bearing system itself. In
one embodiment, an eddy current damper plate 80 is fixedly attached to a
rotatable bearing element. An annular permanent magnet element 82 is
fixedly attached to a stationary bearing element. Element 82 has field
lines that intersect the eddy current damper plate 80. In another
embodiment, eddy current damper plate 80 is fixedly attached to a
stationary bearing element, and annular permanent magnet element 82 is
fixedly attached to a rotatable bearing element. The eddy current damper
plate 80 may have a plurality of concentric conducting rings separated by
material that has a conductivity that is less than that of said conducting
rings. Plate 80 may be made of a non-magnetic metal plate having deep
circular grooves, where only a thin web of supporting material remains on
the surface of the plate that is farthest from the annular permanent
magnet element 82.
During the speed-up of the rotor, resonances may be encountered that could
cause excessive lateral motions of the rotating system in the absence of
damping. One such resonance is the solid-body resonance frequency
associated with the mass of the rotor and the transverse compliance of the
bearing system. Any residual unbalance of the rotating system comprises a
periodic driving force. When the frequency of this driving force coincides
with the solid-body resonance, the amplitude of the vibration may build up
to unacceptable high levels unless damping means are provided. When
conventional eddy-current dampers are employed, deleterious side-effects
may appear, including loss-producing torques, that absorb energy from the
rotor. This improved eddy-current damper assembly maximizes the desired
damping effects on transversely directed vibrations, while minimizing the
torque drag and other undesired effects that may be associated with the
use of conventional eddy-current dampers.
The function of the circular conducting rings is to insure that the eddy
currents that are induced flow only in the azimuthal direction. In the way
the damping force that is exerted on the rotating magnet (when its center
is displaced radially as a result of the resonance), being perpendicular
to the direction of the current, is directed nearly purely radially
inward, i.e., with greatly reduced accompanying torque. In this way the
damper accomplishes its objective; the damping of transverse vibrations
with a minimum of undesirable effects. A model of this damper has been
constructed, tested and found to perform in a far superior manner to
conventional eddy-current dampers.
FIG. 5 shows an improved version of a type of modular electromechanical
battery. The purpose here is to describe a much-simplified version, one
that is particularly well suited for stationary applications, where low
cost, high reliability, and long service life are the critical parameters.
The improved version involves a major simplification of the magnetic
bearing, particularly its elements within the vacuum chamber. This change
in bearing design results in an overall design that permits the
simplification of all the elements of the E-M battery that operate within
the vacuum housing.
Referring to FIG. 5, it can be seen that the upper, non-rotating, element
90 of the magnetic bearing exerts its attractive force through the surface
of the vacuum housing 92 (which may be made of insulating material or
stainless steel) on the lower, rotating element 94. This (lower) element
94 can be of very simple form, consisting only of an appropriately shaped
iron (or ceramic ferrite) pole, for example as shown on the drawing,
possibly with auxiliary elements. In addition, the upper elements
described in FIGS. 1-3 could replace the upper element 90 outside of
vacuum housing 92. The lower bearing elements described in FIGS. 1-3 would
then be placed inside the vacuum housing 92 opposite their corresponding
upper bearing element.
The magnetic bearings of the present invention are stable (strongly
restoring) for radial displacements, but must be stabilized vertically
(axially). Several options for this vertical stabilization have been
described. One type includes a self-adjusting magnetic bearing system in
which magnetic fields arising from a rotating multi-polar element inside
the chamber (co-located with the attracted pole assembly) produce a torque
on a conducting surface above the chamber wall. This torque may then
actuate, either mechanically, or through a spring-loaded switch or
potentiometer, electronic means that adjust the strength of the attractive
pole so as to maintain vertical stability. Alternatively, an ac-activated
circuit that senses the reluctance of the magnetic circuit comprised of
the lower and upper poles could be employed, as a part of an electrical
servo system that would maintain vertical stability.
To allow start up of the rotor, and/or to facilitate shipping of the
completed unit, one or more conventional mechanical bearings that would
disengage upon the lift-off of the rotor assembly at start up, should be
provided. These same mechanical bearings could be used to restrain the
rotor in the case of seismic events.
In FIG. 5, within the vacuum enclosure 92, the remaining rotating elements
of the module are a shaft 95 connected to a rotor 96 and a cylindrical
Halbach array permanent magnet assembly 98 that produce the rotating field
of the generator/motor assembly 99. The vacuum housing itself consists
only of the chamber 92 and the re-entrant ceramic or glass-ceramic
cylinder 100 through which the rotating field is inductively coupled to
the windings that lie inside cylinder 100, thus outside of the evacuated
region. In this way all of the "complicated" elements of the module are
both non-rotating and located outside the evacuated region, leading to a
maximal simplification of the module relative to prior art designs. The
design also has the virtue of removing virtually all sources of heat from
the evacuated region, thus eliminating the tedious issue of providing
cooling means within the chamber itself.
Changes and modifications in the specifically described embodiments can be
carried out without departing from the scope of the invention, which is
intended to be limited by the scope of the appended claims.
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